(1) Field of the Invention
This invention relates generally to image processing and relates more particularly to methods to zoom digital images.
(2) Description of the Prior Art
Either still images or time-varying image sequences are the input to digital imaging systems. A digital zoom capability uses algorithms and zooms in on the digital image in the camera. This can greatly improve the zoom capability of the camera and is less expensive than high-sophisticated zoom lenses, but also loses some image quality. The quality depends upon the algorithms used for zooming.
The challenge of the designers of digital zooming systems is to achieve fast performing methods for zooming providing good quality images and which are easy to be implemented.
There are various patents known dealing with said problems:
U.S. Patent (U.S. Pat. No. 6,101,235 to Zavaljevski et al.) describes methods and apparatus for altering the spatial characteristics of a digital image collected in a CT system using a real-time magnification algorithm. In accordance with one embodiment of the algorithm, once the digital image is collected, an enlarged image is generated by interpolating and filtering the original digital image data. In one aspect, two pass linear interpolation and one-dimensional filtering is utilized to generate the enlarged image. The first pass performs interpolation in the x direction and the second pass performs interpolation in the y direction. The interpolations in the x and y directions are performed in two steps. Initially, linear interpolation is applied to the original digital image to generate interpolated data. A one dimensional convolution filter is then applied to the interpolated data. Utilizing this process, a high quality enlarged image may be generated for any integer zoom factor. In another aspect of the present invention, a two-step procedure of interpolation and two-dimensional filtering is utilized. The first step performs bilinear interpolation of the digital image to generate interpolated data. The interpolated data is then filtered by a two-dimensional convolution filter.
U.S. Patent (U.S. Pat. No. 5,602,870 to Hailey et al.) discloses a spatial interpolation unit used to alter a digital image by performing a zoom operation thereon. Described is a spatial interpolation unit, which comprises a discrete set of filters for which at least some of the cut-off frequencies are logarithmically spaced.
U.S. Patent (U.S. Pat. No. 5,307,167 to Park et al.) describes a digital zoom system utilizing image buffers and employing an approximated bilinear interpolation method. The zoom system includes an analog-to-digital (A/D) converter for converting an analog image signal to a digital image signal, an image buffer for storing the digital image signal generated for said A/D converter, a horizontal operation circuit for interpolating the digital image signal taken along with a horizontal direction, one or more horizontal image buffers for storing the digital image signal for 1-horizontal lines magnified in a horizontal direction, a vertical operation circuit for interpolating said horizontally magnified image signal in a vertical direction, a timing controller for controlling the image buffer, said horizontal and vertical operation circuits, and a digital-to-analog (D/A) converter for converting the digital image signal finally magnified in the horizontal and vertical direction back to analog image signal.